1. Field of the Invention
The invention relates to an MR method in which the nuclear magnetization in an object to be examined and present in an examination zone is excited by at least one magnetic RF pulse, in which the MR signals subsequently received by an MR coil are used to form at least one MR image, and in which at least one microcoil, tuned to the MR frequency, is introduced into the object to be examined. The invention also relates to an MR device for carrying out this method.
2. Description of Related Art
A method and a device of this kind are known from Application Ser. No. 08/754,360. Therein, the microcoil ifs connected to a receiver. The position of the microcoil can be determined from the MR signals received thereby after the excitation by an RF pulse; for example, this position can be superposed on an image formed by means of another MR coil. However, this method involves the risk that the high frequency pulses induce voltages in the connection leads which connect the microcoil present in the object to be examined to the MR receiver, which voltage may cause burning of said object in the vicinity of the connection leads. This risk cannot be eliminated by detuning or deactivating the microcoil during the RF pulse, as is customary for these methods.
It is an object of the present invention to provide a method of the kind set forth in which the described overheating can be avoided. Moreover, the method should be suitable for a larger variety of applications.
This object is achieved according to the invention in that use is made of a microcoil without connection leads and that the MR signals are processed in such a manner that the local variation, induced by the microcoil, of the magnetic field which occurs in the object to be examined due to the RF pulses becomes visible in the MR image.
The invention utilizes the fact that the magnetic field associated with the RF pulses is increased in the direct vicinity of the microcoil (being the area enclosed by the microcoil and the area outside the microcoil in which essentially the magnetic field of the coil is concentrated). Therefore, the nuclear magnetization in the area of the microcoil is influenced to a different extent in comparison with the remainder of the examination zone, and this phenomenon can be made visible in an MR image. Because of the absence of the connection leads, in contrast with the previously mentioned device the MR signals induced into the microcoil cannot be conducted to the MR receiver. Whereas the microcoil of the known device thus is operative only in the receiving phase, the microcoil according to the invention operates only in the transmission phase.
It is to be noted that an article by M. Burl et al., SMR 95, already discloses a xe2x80x9cfiducial markerxe2x80x9d which consists of a microcoil and a sample holder which is situated inside the coil and contains water doped with Gd-DTPA. Because of this doping, a very short T1 relaxation time occurs. These markers are used, for example as reference points for frameless stereotaxy.
In contrast therewith, the microcoil according to the invention is used only within the object to be examined, i.e. without a sample holder; it interacts exclusively with substances or structures of the object to be examined.
A preferred further version is based on the recognition of the fact that in its direct vicinity the microcoil not only varies the amplitude of the magnetic field but also the phase thereof. In the case of a very small Q, for which the increase of the magnetic field can hardly be demonstrated in the MR image, substantial phase variations which become visible in the phase image can still occur.
A further version enables improved identification of the macrocoil. A simple microcoil produces a local increase of the magnetic field which could be hardly noticeable in an MR image, particularly if the microcoil is very small. In the coil segments of different orientation, for example of different winding direction, however, deviating phase positions, and possibly amplitudes, of the magnetic field occur which can be readily observed as a characteristic pattern in an MR image.
A first possible application of the method according to the inventions is to a medical instrument, for example a catheter.
An application of microcoils which was unknown thus far is an application for flux visualization, for example for visualizing the blood flow in blood vessels. For example, when a microcoil is introduced into a blood vessel, the nuclear magnetization present within the microcoil at the instant of application of the excitation RF pulse is excited. This blood volume, however, moves together with the blood flow and, therefore, its position in the MR image is dependent on the temporal distance between the RF pulse and the reception of the MR signal or signals wherefrom the MR image is derived. This embodiment, therefore, enables examination of the propagation of the blood volume.
In a further version the projection direction preferably extends perpendicularly to the flow direction, and represents a very fast examination method.
A further possible application again utilizes the fact that the magnetic field is increased essentially only within the microcoil. When the amplitude of the RF pulses is suitably chosen, only the area within the microcoil will experience an excitation which suffices for an MR image, so that the MR image can be limited to this small area; this results in short measuring times and also enables fluoroscopic applications.
A still further version defines the possibility of using the microcoil for hyperthermia for which it is advantageous that the necessary temperature measurement can also be performed in known by means of an MR method.
The invention also includes an MR device for carrying out the MR method.